An internal combustion engine may be provided with one or more turbochargers, which are a type of forced induction system, for compressing a fluid (air or fuel/air mixture) supplied to one or more combustion chambers within corresponding combustion cylinders, thus boosting the engine's capability without significantly increasing weight. Turbochargers extract heat energy from the exhaust flow from the engine, and convert that heat energy into power to rotate a turbine, which in turn drives a compressor.
One problem with turbochargers is that they often do not provide an immediate air boost when the vehicle operator depresses the accelerator pedal, so-called “turbo lag” or boost lag. One way to decrease turbo lag is to reduce the size of the turbine stage. However, a smaller turbine (“turbo”) stage with lower rotating inertia, while improving transient response, may be unable to produce the mass flow required at higher engine speed. A larger turbocharger can produce the mass flow required at high engine speed, but may experience greater turbo lag because the inertia of the rotating parts is greater, so it takes longer to accelerate to its required speed.
Dual-turbo systems have been designed to solve the above problem. A dual-turbo system can be a parallel dual-turbo system, a sequential dual-turbo system, or a parallel-series dual-turbo system. The parallel dual-turbo system has two small turbos, one being active across the entire engine speed range with the other becoming active only at higher engine speed. The sequential dual-turbo system uses two turbochargers of different sizes. The smaller turbocharger, with lower inertia, spins up to speed very quickly, reducing lag and improving low-end boost, while the larger turbocharger can be activated at higher speeds to provide more flow capacity. The parallel-series configuration has valving which can vary the turbine flow to either turbine, in either a parallel configuration, or a series configuration. This is known as a Regulated Two Stage (R2S) system. In any of these configurations the compressor stages can be parallel, sequential, series, or a combination of any configuration.
In emission controlled diesel engines, catalytic aftertreatments can be used to meet emission standards. The pollutant-converting catalysts typically require a minimum temperature to start up and work effectively. The two-stage turbocharging systems can provide high boost and reduce turbo lag. However, after the exhaust goes through both turbine stages of the two-stage turbocharging, it has cooled down significantly depending on how much boost is being made and how much work is being extracted from the exhaust. Another factor is the thermal inertia of the two-stage turbine castings, which can consume heat energy otherwise useful to the aftertreatment. This can cause difficulty for the catalyst to “light off” at initial startup and maintain a proper working temperature at idle and very low power conditions.
In U.S. Patent Application Publication No. US 2007/0119168 A1 to Turner, a turbocharged internal combustion engine is described as shown in FIG. 1 having a two-stage charging system. Each cylinder has two exhaust valves “a” and “b”, where each valve “b” can be opened independently of the valve “a.” The exhaust valves “a” of all cylinders are connected to a first exhaust duct (104) which leads the exhaust gases to the turbine part (105A) of a high pressure turbocharger (105) and the exhaust valves “b” of all cylinders are connected to a second exhaust duct (106) through which the exhaust gases flow to the turbine part (107A) of a low pressure turbocharger (107), bypassing the high pressure turbocharger (105). The exhaust gases exiting the turbine (107A) of the low pressure turbocharger (107) pass through an exhaust passage (110) to atmosphere via a catalytic converter (111). Although the high pressure turbocharger (105) may be bypassed on start-up of the engine to reduce the time to light off the catalyst (111), this does not solve the problem of delayed “light off” at initial startup because the exhaust gases may not have high enough temperature to light off the catalyst immediately after passing through the low pressure turbocharger (107).
Turner also discloses another embodiment as shown in FIG. 2 in which the exhaust gases can bypass the single turbocharger (15) and flow directly to the starter catalytic converter (17) and then flow through the main catalytic converter (21). This system of Turner, however, does not provide a two-stage turbocharging system which can provide the necessary boost while reducing turbo-lag. This system of Turner also requires two separate catalytic converters, which increases the cost of the system, complicates the flow path, and occupies additional space in an engine compartment that typically has little space to spare.
In FIG. 3, U.S. Pat. No. 4,444,012 to Gauffres discloses an exhaust pipe arrangement for internal combustion engines similar to the embodiment shown in FIG. 2 described above. As shown in FIG. 3, in the Gauffres exhaust pipe arrangement, a bypass conduit (7) branches off from the exhaust pipe (1) and bypasses or circumvents the exhaust gas turbocharger (3). Different from the Turner embodiment shown in FIG. 2, in the Gauffres arrangement the bypass conduit (7) terminates in the exhaust pipe at a position downstream of the starter catalyst (4) and the oxygen sensor (5) as controlled by a blow off valve (8). This system of Gauffres, however, also does not provide a two-stage turbocharging system which can provide the necessary boost while reducing turbo-lag. This system of Turner also requires two separate catalytic converters, which increases the cost of the system, complicates the flow path, and occupies additional space in an engine compartment that typically has little space to spare.
Thus, there is a need for a multi-stage turbocharging system that facilitates the immediate light off of the catalyst at the initial startup of the engine. There is a further need for such a system that is cost-effective, space-efficient, and/or reliable.